In this paper I argue that if one is to do justice to reason's unity in Kant, then one must acknowledge that reason's practical ends are presupposed in every theoretical investigation of nature. Thus, contrary to some other commentators, I contend that the notion of the metaphysical ground of the unity of nature should not be attributed to the “dynamics of reason” and its “own practical purposes.” Instead, the metaphysical ground of the unity of nature is in fact an indispensable and necessary notion for reason in both its theoretical and practical functions, but this need of reason to presuppose such a notion can only find its adequate proof in the practical. By offering a synopsis of Kant's accounts of nature's systematicity in the Transcendental Ideal of the Critique of Pure Reason (Part I), the Appendix to the Dialectic of the Critique of Pure Reason (Part II), and in the Critique of Judgment (Part III), I identify in each section Kant's theoretical and practical arguments for reason's presupposition of the “unconditioned,” demonstrate their structural interdependence, and show a general continuity in Kant's position on this issue throughout his critical system.